Wikileaks has said it plans to release a video of a US air strike in Afghanistan which allegedly killed many children. Photograph: public domain

The whistleblowing website WikiLeaks says it plans to release a secret military video of one of the deadliest US air strikes in Afghanistan in which scores of children are believed to have been killed.

WikiLeaks announced the move in an email to supporters. It said it fears it is under attack after the US authorities said they were searching for the site's founder, Julian Assange, following the arrest of a US soldier accused of leaking the Afghanistan video and another of a US attack in Baghdad in which civilians were killed.

WikiLeaks released the Baghdad video in April, prompting considerable criticism of the US military. It says it is still working to prepare the film of the bombing of the Afghan village of Garani in May 2009.

The Afghan government said about 140 civilians were killed in Garani, including 92 children. The US military initially said that up to 95 people died, of which about 65 were insurgents. However, American officials have since wavered on that claim and a subsequent investigation admitted mistakes were made during the attack.

The video could prove to be extremely embarrassing to the US military and risks weakening Afghan support. The US said it was targeting Taliban positions when it used weapons that create casualties over a wide area, including one-tonne bombs and others that burst in the air. But two US military officials told a newspaper last year that no one checked to see whether there were women and children in the buildings.

The US commander, General David Petraeus, said a year ago that the military's video of the attack would be made public as evidence that the US assault on Garani was justified. But it was not released.

In an email to supporters, Assange said WikiLeaks has the Garani video and "a lot of other material that exposes human rights abuses by the US government".

Last week, it was revealed that US authorities are trying to make contact with Assange to press him not to publish information the Pentagon says could endanger national security. Assange cancelled an appearance in Las Vegas last Friday.

In his email, Assange also calls on supporters to protect the website from "attack" by the authorities following the detention of a US soldier, Bradley Manning, who was arrested in Iraq after admitting to a former hacker that he leaked the Garani and Baghdad videos to WikiLeaks.